As described in an article by Alan Podell, entitled "A High Directivity Microstrip Coupler Technique" that appeared in the May, 1970, issue of IEEE G-MTT, and in an article by M. D. Tremblay entitled "Design of High Directivity Microstrip Quarter Wavelength Directional Coupler with Small Coupling Coefficients", that appeared in the Aug. 6, 1973 issue of Bell Laboratories, directive microstrip couplers may be comprised of a substrate of insulating material having a first conductive area, which serves as a ground plane, on one side and second and third patterned conductive areas on the other. The outside edges of the second and third areas are straight parallel lines, and the adjacent edges are spaced parallel "wiggly" paths in the form of sawteeth. A terminal is located at the end of the second and third areas.
In operation of the coupler, microwave energy coupled to a terminal at one end of the second area travels to the terminal at the other end of that area with part of the energy coupled to an output terminal at one end of the third area. The remaining terminal of the third area is connected to ground via a resistive characteristic impedance. Microwave energy propagates along the microstrip areas in two modes, an even mode that travels equally distributed along the inner and outer edges, and an odd mode that propagates primarily along the adjacent edges that are in the form of sawteeth. The odd mode travels faster than the even mode, but both arrive at the output terminal at the same time because of the length of the sawtooth paths is greater than the length of the straight line paths.
One disadvantage of the directional microwave coupler just described is that the large current density at the discontinuity points or corners of each saw tooth causes relatively large resistive losses. Another disadvantage is that RF energy is radiated at these same points, particularly at high microwave or millimeter wave frequencies.